Not Applicable.
Not Applicable.
1. Field of Invention
The present invention relates generally to vacuum electronics and, more particularly, to circuits and methods for linearizing vacuum electronic amplification.
2. Description of the Background
Amplifiers come in many forms and are used in many applications. For example, amplifiers may be used with digital or analog signals, may be used in communications systems such as wireless telecommunications and satellite communications systems, and may be semiconductor-based or vacuum tube-based.
The performance demanded of amplifiers continues to increase, and many conventional amplifiers are failing to keep pace. For example, conventional semiconductor microwave amplifiers lack the power capabilities required by many modem microwave systems. As a result, vacuum tube power amplifiers are essential components of many modern microwave systems, including telecommunications, radar, electronic warfare, and navigation systems, because microwave tube amplifiers can provide microwave energy at levels of power higher by orders of magnitude in comparison to semiconductor microwave amplifiers. The higher power levels offered by tube devices are facilitated by the fact that electrons can travel at a much higher velocity in a vacuum than in a semiconductor. The higher velocity permits use of larger structures with the same transit time. Larger structures, in turn, permit greater power levels.
The power amplification for modern high power vacuum electronic microwave amplifiers (VEMAs), however, is typically non-linear. For instance, phase non-linearity may be caused when the electrons slow down while moving through an interactive region of the tube. That slowing is a result of the electrons losing kinetic energy as they amplify a signal passing through the tube. At high power levels, however, the electrons start to slow down significantly and desynchronize from the RF field in the interactive region, thereby causing a phase lag between the input and output signals. For example, without phase compensation, the phase delay of the output signal for a traveling wave tube (TWT) VEMA operating at its saturation point may be as great as 70xc2x0-80xc2x0, which may be unacceptable for many applications, such as digital communications. Moreover, at high power levels, the beam current is not large enough to continue amplifying the input signal, causing an RF saturation or an amplitude non-linearity. Thus, non-linearity in power amplification generates higher-order intermodulation products, which may result in undesirable spectral regrowth in adjacent channels, and phase distortions, which in turn may increase bit errors in digital communications systems. The drawbacks are especially acute in digital wireless communications systems where multiple communication signals are typically multiplexed onto a single, narrow wavelength-band channel due to the limited RF spectrum bandwidth. Accordingly, practical communication limitations as well as government standards require minimal higher-order intermodulation and phase distortion. For instance, for PCS cellular telephony networks, the higher order intermodulation signals have to be 60 dB below the carrier signal.
One technique to minimize power amplification non-linearity is to operate a VEMA only in its linear range, which typically is a small fraction of its power capacity. For example, for an application that requires 50 Watts of amplification power, a tube capable of 500 Watts of amplification power may be required to produce an adequate linear range. This technique is inefficient and expensive.
Accordingly, there exists a need for an efficient and inexpensive technique for improving the linearity of high power vacuum electronic microwave amplifiers.
The present invention is directed to an amplifier system. According to one embodiment, the amplifier system includes a vacuum tube amplifier, an electron source voltage modulation circuit, and a linearization circuit. The vacuum tube amplifier includes an input signal terminal and an output signal terminal. The electron source voltage modulation circuit includes an input terminal coupled to one of the input signal terminal and the output signal of the vacuum tube amplifier, and includes an output terminal coupled to one of an electron source of the vacuum tube amplifier and an interaction region of the vacuum tube amplifier. The linearization circuit is coupled to the input signal terminal of the vacuum tube amplifier.
According to another embodiment, the present invention is directed to an amplifier system, including a vacuum tube amplifier, an electron beam control circuit, and a linearization circuit. The vacuum tube amplifier includes an input signal terminal, an output signal terminal, and an electron source. The electron beam control circuit includes an input terminal coupled to one of the input signal terminal and the output signal of the vacuum tube amplifier, and includes an output terminal coupled to the electron source of the vacuum tube amplifier. The linearization circuit is coupled to the input signal terminal of the vacuum tube amplifier.
According to yet another embodiment, the present invention is directed to an amplifier system, including a vacuum tube amplifier, an electron source voltage modulation circuit, an electron beam control circuit, and a linearization circuit. The vacuum tube amplifier includes an input signal terminal and an output signal terminal. The electron source voltage modulation circuit includes an input terminal coupled to one of the input signal terminal and the output signal of the vacuum tube amplifier, and includes an output terminal coupled to one of an electron source of the vacuum tube amplifier and an interactive region of the vacuum tube amplifier. The electron beam control circuit includes an input terminal coupled to one of the input signal terminal and the output signal of the vacuum tube amplifier, and includes an output terminal coupled to the electron source of the vacuum tube amplifier. The linearization circuit is coupled to the input signal terminal of the vacuum tube amplifier.
In contrast to prior techniques, the present invention provides an efficient and inexpensive technique for linearizing the power amplification of vacuum tube amplifiers. The present invention provides an advantage over the prior techniques because, according to one embodiment, it provides for the dynamic compensation of the deleterious effects caused by non-linear power amplification of a vacuum tube amplifier based on the power, or envelope, of the input signal to the amplifier, thereby permitting the use of less expensive and more efficient components. Moreover, when combined with the linearization circuit, the present invention permits the suppression of nonlinear intermodulation signals generated by the vacuum tube amplifier by 60 dB or more below the carrier signal. These and other benefits of the present invention will be apparent from the detailed description of the invention hereinbelow.